The most recent recommendations on the protein requirements of elderly human subjects are extrapolated from data obtained with young adults. The objective of the proposed research work is to directly estimate the protein requirement of elderly men and women and to evaluate the adequacy of the current recommendations for protein intake in this age-group. Nitrogen balance studies will be conducted to determine (a) obligatory N losses and (b) the relationship between these latter losses and the minimum amount of high quality protein required to achieve satisfactory N balance in this age group. Because we have conducted essentially identical studies in young adults a meaningful comparative evaluation of protein requirements in young adults and elderly subjects will be possible. In addition, studies will be conducted to assess whether whole body N turnover and rates of body protein synthesis and catabolism are different in elderly subjects as compared with young adult subjects. The purpose of this phase is to provide a knowledge of the characteristics of protein metabolism in elderly subjects as well as to begin to define the metabolic basis of the protein requirement of older people.